Belgian post

My insurance company has sent me a certified letter (aangetekende brief) with valuable information, something they told me in advance. This weekend I wanted to collect the letter at my local post-office branch (which is housed in a grocery store). But they refused to give me the letter, which addresses my company, as I did not have any proof that I was in fact authorized to accept mail for my company. Fair enough, no problem, I should have known.